Could you make a solar sail that derives it's energy from the decay of a radioactive element such as plutonium, uranium, or palonium? I understand that such elements radiate in all directions but if you had a 1 kg sphere of it attached to a boon which was attached to a reflective sail (e.g. aluminum) would you not be able to harness the total momentum generated by the alpha, beta, or gamma particles? If the alpha particles are absorbed rather than reflected then...couldn't you encase the radioactive element in a lead casing, allowing it to heat and emit it's energy via infrared to the solar sail? I understand that gamma will likely sail on through but most of the energy released in decay is from alpha particles which are easily blocked. My estimate is that you could get up to 1/1000th the speed of light from the energy that is released over a 100 day decay of palonium (140 W/gram). This wouldn't be worthwhile for interstellar travel but you could zip around the solar system quite nicely. The thrust should be about 50X that of the Sun's radiation pressure near earth but if you need more you could simply increase the size of the 1Kg sample.